I thought I read somewhere that the resistance(DCR?) of the TVC changes according to setting. That's why some recommend not using one after a split riaa network. If this is true, wouldn't you be creating a variable x-over?

Sorry, maybe I used the wrong term. I think it was Allen Wright who said the S&B's input impedance changes with attenuation setting, therefore making them unusable when placed directly after his phono section(needs to see a consistent 50K(?) load).

Well, it's a tricky one - I wasn't expecting too many replies as it's something that I've never seen suggested before; it's a bit at the fringe...

Frank - you're right, it's an expensive way of building a filter, but if the TVCs are already bought and paid for...

The main issues I can see are:

1) Following stage must have very high impedance
2) The inductance of the TVC is a function of frequency and level.

As regards (2), that's what air-gaps are for. Dunno how much of a gap S&B use though...

Now if we are filtering the audio signal -12db/octave at 120Hz, our ac flux on the core is much lower, which should reduce (already low) distortion, but I was thinking that this might give us some headroom to apply a tiny dc bias current to the primary (say a few microamps) which I was thinking might help linearise the inductance. I'm guessing the B-H curves are unlinear around zero crossing, so we'd get away from that. Much like gapped SE vs ungapped PP transformers.

Wish I had the test gear to check this out...

And don't forget, in addition to getting a filter for "free" it means we can use a lower value, higher quality cap.

Originally posted by analog_sa Seems like there is no airgap. Which is of course a pity. Do you really need 400H? 40 would do me fine and i bet it would sound better too.

cheers

peter

Hi Peter,
I agree with you there; the TVC does have 2 primary windings which can be wired in parallel to give a quarter of the inductance - but it would be nicer to achieve 100H with a bigger air-gap...

The TVCs are on loan to a friend at the moment; when I get them back I'll see if Ican make some crude measurements of L vs level and frequency.

but I was thinking that this might give us some headroom to apply a tiny dc bias current to the primary (say a few microamps) which I was thinking might help linearise the inductance. I'm guessing the B-H curves are unlinear around zero crossing, so we'd get away from that. Much like gapped SE vs ungapped PP transformers.

its interesting to note that looking at the curves in the old texts for perm vs frequency for various values of H (dc offset) that small amounts of H while reducing the perm, do make the perm much more linear, which seems to be a very similar behavior to what an airgap does.